1. Field of the Invention
The present invention relates to a method for enhancing the dielectric strength of an electrical power cable and, more particularly, relates to an efficient and effective method for introducing a dielectric property-enhancing fluid into the interstitial void volume of an electrical cable segment.
2. Description of the Related Art
Extensive networks of underground electrical cables are in place in many parts of the industrialized world. Such underground distribution offers great advantage over conventional overhead lines in that it is not subject to wind, ice or lightning damage and is thus viewed as a reliable means for delivering electrical power without obstructing the surrounding landscape, the latter feature being particularly appreciated in suburban and urban settings. Unfortunately, these cables, which generally comprise a stranded conductor surrounded by a semi-conducting shield, a layer of insulation jacket, and an insulation shield, often suffer premature breakdown and do not attain their originally anticipated longevity of 30 to 40 years. Their dielectric breakdown is generally attributed to at least two so-called “treeing” phenomena which lead to a progressive degradation of the cable's insulation. The first, “electrical treeing,” is the product of numerous electrical discharges in the presence of strong electrical fields which eventually lead to the formation of microscopic branching channels within the insulation material, from which the descriptive terminology derives. A similar mechanism, “water treeing,” is observed when the insulation material is simultaneously exposed to moisture and an electric field. Although the latter mechanism is much more gradual than electrical treeing, it does occur at considerably lower electrical fields and therefore is considered to be a primary contributor to reduced cable service life. Since replacing a failed section of underground cable can be a very costly and involved procedure, there is a strong motivation on the part of the electrical utility industry to extend the useful life of existing underground cables in a cost-effective manner.
Two early efforts by Bahder and Fryszczyn focused on rejuvenating in-service cables by either simply drying the insulation or introducing a certain liquid into the void volume associated with the conductor geometry after such a drying step. Thus, in U.S. Pat. No. 4,545,133 the inventors teach a method for retarding electrochemical decomposition of a cable's insulation by continuously passing a dry gas through the interior of the cable. Only nitrogen is explicitly recited as the gas to be used and maximum pressure contemplated for introducing the gas is 50 psig (pounds per square inch above atmospheric pressure). Not only is this method cumbersome, but it requires extensive monitoring and scheduled replenishment of the dry gas supply. U.S. Pat. No. 4,372,988 to Bahder teaches a method for reclaiming electrical distribution cable which comprises drying the cable and then continuously supplying a tree retardant liquid to the interior of the cable. The liquid was believed to diffuse out of the cable's interior and into the insulation, where it filled the microscopic trees and thereby augmented the service life of the cable. This disclosure suffers from the disadvantage that the retardant can exude or leak from the cable. The loss of liquid was addressed by a preferred embodiment wherein external reservoirs suitable for maintaining a constant level of the liquid were provided, further adding to the complexity of this method.
An improvement over the disclosure by Bahder was proposed by Vincent et al. in U.S. Pat. No. 4,766,011, wherein the tree retardant liquid was selected from a particular class of aromatic alkoxysilanes. Again, the tree retardant was supplied to the interstices of the cable conductor. However, in this case, the fluid can polymerize within the cable's interior as well as within the water tree voids in the insulation and therefore does not leak out of the cable, or only exudes therefrom at a low rate. This method and variations thereof employing certain rapidly diffusing components (see U.S. Pat. Nos. 5,372,840 and 5,372,841) have enjoyed commercial success over the last decade or so, but they still have some practical limitations when reclaiming underground residential distribution (URD) cables, which have a relatively small diameter, and therefore present insufficient interstitial volume relative to the amount of retardant required for optimum dielectric performance. Thus, although not explicitly required by the above mentioned disclosures, a typical in-the-field reclamation of URD cables employing such silane-based compositions typically leaves a liquid reservoir connected to the cable for a 60 to 90 day “soak period” to allow sufficient retardant liquid to penetrate the cable insulation and thereby restore the dielectric properties. For example, cables having round conductors smaller than 4/0 (120 mm2) generally require the above described reservoir and soak period to introduce a sufficient amount of treating fluid. In reality, this is an oversimplification, since some cables larger than 4/0 with compressed or compacted strands would suffer from the same inadequate fluid supply. As a result, it is generally necessary to have a crew visit the site at least three times: first to begin the injection which involves a vacuum at one end and a slightly pressurized feed reservoir on the other end, second to remove the vacuum bottle a few days later after the fluid has traversed the length of the cable segment, and finally to remove the reservoir after the soak period is complete. The repetitive trips are costly in terms of human resource. Moreover, each exposure of workers to energized equipment presents additional risk of serious injury or fatality and it would be beneficial to minimize such interactions. In view of the above limitations, a circuit owner might find it economically equivalent, or even advantageous, to completely replace a cable once it has deteriorated rather than resort to the above restorative methods.
Unlike the above described URD systems, large diameter (e.g., feeder) cables present their own unique problems. Because of the relatively larger interstitial volumes of the latter, the amount of retardant liquid introduced according to the above described methods can actually exceed that required to optimally treat the insulation. Such systems do not require the above described reservoir, but, as the temperature of the treated cable cycles with electrical load, thermodynamic pumping of ever more liquid from the cable's core into the insulation was believed to be responsible for the catastrophic bursting of some cables. This “supersaturation” phenomenon, and a remedy therefor, are described in U.S. Pat. No. 6,162,491 to Bertini. In this variation of the above described methods, a diluent, which has a low viscosity, is insoluble in the insulation and is miscible with the retardant liquid, is added to the latter, thereby limiting the amount of retardant which can diffuse into the insulation. A methodology for determining the proper amount of the diluent for a given situation is provided. While this method may indeed prevent the bursting of large cables after treatment it does not take advantage of the extra interstitial volume by employing a diluent which is incapable of providing any benefit to the long-term dielectric performance of the insulation. Thus, this method does not take advantage of the large interstitial volume associated with such cables.
In all of the above recited methods for treating in-service cables, the retardant liquid is injected into the cable under a pressure sufficient to facilitate filling the interstitial void volume. But, although pressures as high as 400 psig have been employed to this end (e.g., Transmission & Distribution World, Jul. 1, 1999, “Submarine Cable Rescued With Silicone-Based Fluid”), the pressure is always discontinued after the cable is filled. At most, a residual pressure of up to 30 psig is applied to a liquid reservoir after injection, as required for the soak period in the case of URD cable reclamation. And, while relatively high pressures have been used to inject power cables, this prior use is solely to accelerate the cable segment filling time, especially for very long lengths as are encountered with submarine cables (the above Transmission & Distribution World article), and the pressure was relieved after the cable segment was filled. Furthermore, even when higher pressures were maintained in an experimental determination of possible detrimental effects of excessive pressure, the pressure was maintained for only a brief period by an external pressure reservoir to simulate the injection of longer segment lengths than those employed in the experiment (“Entergy Metro Case Study: Post-Treatment Lessons,” Glen Bertini, ICC April, 1997 Meeting, Scottsdale, Ariz.). In this case, even after two hours of continuous pressure at 117 psig, the interstitial void volume of the cable segment was not completely filled and it was suggested that the inability to completely fill the interstices was due to severe strand compaction.